The present invention relates to storage systems for holographic data and images. More specifically, the present invention relates to spatial light modulators for use in storing holographic data and images.
Holography is a lensless, photographic method that uses coherent (laser) light to produce three-dimensional images by splitting the laser beam into two beams and recording on a storage medium, such as a photographic plate, the interference patterns made by the reference light waves reflected directly form a mirror, and the waves modulated when simultaneously reflected from the subject.
In a holographic data/image storage system, the information to be stored is written into the storage medium with a spatially varying light intensity produced by the coherent interference between an information (object) beam and a reference beam. Details of this process are well understood and described in the literature. See, for example, J. Goodman, Introduction to Fourier Optics, Chapter 8 (McGraw-Hill, 1968).
Data is encoded onto the information beam by spatially modulating the intensity of the beam. A common method for intensity modulation is to use a two dimensional array of elements (pixels) in which the properties of the individual pixels are varied to control the ratio of the light transmitted or reflected to that incident on the pixel. Such a device is known as a spatial light modulator (SLM). Methods to achieve these objectives are well known and documented. See, for example, U. Efron (Ed.), Spatial Light Modulators (Dekker, 1994).
FIG. 1 is a cross-sectional view of a portion of a single row of pixels of a liquid crystal reflective spatial light modulator (SLM) of a known type.
The SLM is formed on a silicon substrate 20. Integrated electronics 22 are formed on the silicon substrate using conventional semiconductor planar processes. An element of the integrated electronics 22 corresponds to each pixel of the SLM array. An individual pixel electrode 24 is electrically connected to be driven by a corresponding element of the integrated electronics 22.
Liquid crystal material 32 covers the pixel electrodes 24. A layer of SLM cover glass 38 contains the liquid crystal material. A conducting layer 36 covers the underside of the cover glass 38. An electric field can then be produced across the liquid crystal material at a particular pixel by applying an electric potential between the particular pixel electrode 24 and the conducting layer 36. The electric potential for a particular pixel is controlled by the element of the integrated electronics 22 associated with that pixel electrode 24. The potential across the liquid crystal material 32 at a particular driven pixel electrode 24 causes the liquid crystal material to modulate the light beam at that pixel.
A polarizer 40 polarizes the incoming light beam 51. After being polarized by the polarizer 40, the incoming light beam passes through the SLM cover glass 38 and traverses the liquid crystal material 32. The light beam is reflected by the driven pixel electrode 24, reversing its path as reflected beam 53. The reflected beam 53 passes back through the liquid crystal material and the cover glass 38 to impinge the polarizer 40.
The optical axes of the liquid crystal material 32 are oriented so that the reflected light 53 is polarized orthogonally to the incoming beam 51. The orthogonal polarization of the reflected beam 53 causes the reflected beam 53 to be reflected by the polarizer 40, rather than passing through it. The reflected beam 53 is directed toward the storage medium (not shown).
A liquid crystal alignment layer 34 may be included between the liquid crystal material 32 and the cover glass conducting layer 36. The liquid crystal alignment layer provides alignment to the liquid crystal modulator medium.
Those skilled in the art understand that the performance of a holographic storage system may be improved by randomizing the phase of the information beam. See, for example, J. Hong, et al., "Influence of Phase Masks on Cross Talk in Holographic Memory," Optics Letters, Vol. 21, No. 20, Pp. 1694-96 (Oct. 15, 1996).
A phase mask is typically used to change the phase of the information beam and accomplish such randomization. The phase mask is placed in the path of the reflected modulated beam 53. The phase mask is constructed and aligned in the reflected modulated light beam 53 to provide a particular phase value to each pixel of the modulated information beam. Thus, the distribution of the phase values in the phase mask corresponds to the distribution of the phase of the information beam. To create a particular phase distribution in the information beam, the phase values are distributed across the phase mask in a corresponding pattern. If a random phase distribution pattern is desired in the information beam, phase values may be randomly distributed across the phase mask.
Binary phase values of 0 and .pi. may be distributed in a random manner across the array to generate the randomization of the information beam. Those skilled in the art will recognize that other phase values may be used to generate the appropriate randomization, or that other phase patterns may be desired in the information beam.
Each phase value applied to the information beam by the phase mask must be accurately aligned with the associated pixel of the reflected modulated beam 53. Therefore, the spatial light modulator (SLM) and the phase mask must be carefully aligned. Misalignment of the SLM and the phase mask pixels results in increased cross-talk between detector array pixels during readout of the storage medium. It has been found that in an arrangement with a ten micron pixel pitch, the alignment between the SLM pixels and the phase mask pixels should be kept to about one tenth of a micron.